The present invention relates, in general, to semiconductor devices, and more particularly, to semiconductor devices that are responsive to and sense acceleration forces.
FIG. 1 is an enlarged cross-sectional view of a previously known, uncapped sensor device 10. Sensor device 10 includes a source region 12 and a drain region 13, which are separated by a channel region 15. Channel region 15 is covered with a layer of dielectric material 16 such as silicon dioxide. Sensor device 10 also includes a microstructure 17 that is free to move in response to an acceleration force applied to sensor device 10. Microstructure 17 moves across an airgap 27 in response to an acceleration force and this motion is indicated in FIG. 1 with an arrow 18.
To operate sensor device 10, a voltage potential is placed on microstructure 17, semiconductor substrate 11, source region 12, and on drain region 13 to generate a current flow across channel region 15. This current flow is indicated in FIG. 1 with an arrow 14. When an acceleration or de-acceleration force is applied to sensor device 10, the force causes microstructure 17 to move. This in turn, either increases or decreases the current flow (arrow 14) between source region 12 and drain region 13 a proportional amount.
One problem common to most conventional, uncapped sensor devices is that both their sensitivity to motion and their quiescent drain current tends to vary over time. For example, FIG. 2 is a graph 20 that is provided to illustrate how the current output of sensor device 10 varies over time. A line 23 represents the source-drain current (FIG. 1, arrow 14) along a y-axis 22 in micro-amps (.mu.A) as a function of time along an x-axis 21 in seconds. As shown, the actual source-drain current of conventional uncapped devices initially increases rapidly during operation. This variability in output current makes the performance of a conventional sensor device unpredictable over time.
Up to now, the source of this variability in the performance of conventional sensor devices has not been understood. However, the impact of the variability of source-drain current in sensor devices is significant as it requires complicated and expensive circuitry to compensate for this problem. Accordingly, a need exists to first identify the cause of the variability in the performance associated with conventional sensor devices and then to provide a sensor device that does not suffer from this variability.